The present invention relates to a system of measuring sectional shape of a minute pattern such as a resist or wiring on a semiconductor substrate. More particularly, the invention is concerned with an examination system in which a fast measurement of sectional shape, free from any fluctuation in the measuring object, is attained even from a deteriorated picture.
Hitherto, stereoscopic measurement has been used in various fields and uses such as formation of topographic map by aerial photography, microscopic three-dimensional observation of microscopic structure, and so forth. In this measuring method, much time and labor are required for the manual analysis of the data, therefore studies have been made for developing automatic analysis by computers. An example of such study is described in detail in "Three-Dimensional Analysis of Stereoscopic Image" by Tetsuo Kobori et al in INFORMATION PROCESSING Vol. 22, No. 9 pp. 846-855 (1981).
FIGS. 1(a) and 1(b) show, by way of example, a conventional method for detecting pictures of a specimen 2, in which the picture is sensed by an optical microscope 1 while a table 3 (see FIG. 1(a)) mounting the specimen 2 is tilted as indicated by 3' in FIG. 1(b).
FIGS. 2(a) and 2(b) show the pictures obtained by the method corresponding to FIGS. 1(a) and 1(b). These pictures are shown in the form of equi-altitude lines, so that the difference in altitude between two points marked at x and .DELTA. can be known if the positions of these points are identified. In manual measurement, the operator inputs the coordinate values of the above-mentioned points by a known measure such as, for example, cursor, so that the value of the altitude difference can be obtained automatically.
When carrying out automatic measurement, it is necessary to locate the point appearing in FIG. 2(a) on the picture shown in FIG. 2(b).
Hitherto, this could be conducted by a method which will be explained hereinunder with reference to FIGS. 3(a) and 3(b). Namely, an area 4 of a certain size is defined in the picture shown in FIG. 3(a) around the point to be located, and a search area 5 which is considered to involve the point corresponding to the above-mentioned point in FIG. 3(a) is selected in the picture shown in FIG. 3(b). Usually, the search area 5 is determined to be somewhat greater than the window area 4 defined on the picture shown in FIG. 3(a). Then, the correlation between the window area 4 and the search area 5 is obtained, and the point shown in FIG. 4 which exhibits the maximum correlation value is determined as the corresponding point. This series of operations is referred to as "template matching", and is usually conducted through digital processing. Representing the number of picture elements (pixels) corresponding to one side of the window by N, the window size can be expressed by N.times.N in terms of the number of picture elements.
The gray level of the picture element (i,j) in FIG. 3(a) is expressed by Aij, while the gray level of the picture element (i',j') in FIG. 3(b) is expressed by Bi'j'. The correlation value between the window 4 around the picture element (i,j) in the picture shown in FIG. 3(a) and the search area 5 of the picture element (i',j') in the picture shown in FIG. 3(b) is expressed by the following formula (1) even in the simplest case. ##EQU1##
The computation of this formula requires multiplication N.sup.2 times. Assuming that the size of the search area is expressed by M.times.M in terms of the number of picture elements, the correlation computation requires multiplication M.sup.2 times. Consequently, multiplication N.sup.2 .times.M.sup.2 times is required for obtaining correlation value between corresponding picture elements in the window 4 and the search area 5.
In a typical case of N=20 and M=100, multiplication 4,000,000 times is required to the desired template matching.
It is evident that template matching to determine the desired value requires considerable time. When the altitude difference is to be computed only with respect to the points and shown in FIGS. 3(a) and 3(b), the computation is affected by local deformation in the portion where the point exists. In addition, this conventional method inevitably suffers from problems such as instability due to noise produced by, for example, fluctuations in the acceleration voltage of the electron microscope.
The stereoscopic measurement described hereinbefore relies upon the principle of parallax. This principle is described in detail in, for example, "Digital Picture Processing", 2nd edition, by Azriel Rosenfeld & Avinash C. Kak, Published by Academic Press, 1982, Vol. 2, pp 29-37. An example of the simple application of this principle is shown in IBM Technical Disclosure Bulletin Vol. 26, No. 1, June 1983, pp 189-190.